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Aviation History
1914
1914 - 0146.PDF
QFygflT concentrated and balanced round the bird's centre of gravity. The " bump " on the wing balances the downward curve of the body. This means that when the tail is folded a bird has no "weather cock directional stability" ; a gust of wind striking the bird sideways would therefore tend to bring the bird with it. When flapping, the bird instinctively corrects this by the power of the wing alone. In gliding, however, the expanded tail oscillates so as to convert itself into a vertical fin, which tends to hold the bird into the wind and preserve direction. 48. I shall now describe briefly the controls which I advocate. (&) Fig. 1. Fig. \a shows in plan form wings of the character suggested, while Fig. \b shows a section through the single spars or arms on which the wings are mounted. The wings are mounted on central spars which can rotate round a horizontal axis, so that the angles of incidence of the wings may be decreased or increased at will. In my early models the wings were mounted independently, but as the result of more recent experiments I now connect them so that alteration of the angles of incidence is simultaneous for both wings. In plan form the entire trailing edge is roughly a semi-ellipse struck on a major axis equal to the span selected and a semi-minor axis carefully proportioned to the span. The leading edges of the wings are parabolic curves struck on bases equal to two-thirds of the span commencing from the tip of each wing. The two vertices are, therefore, one-third of the total span from each tip. The heights of the parabola: are also carefully proportioned to the span. The spars are centred along the dotted line shown in the figure, or at one-third of the chord measured from the leading edge at the vertices of the parabola:. The lower line of the section of the spar (Fig. lb) is for the lower or under surface of the wing the line which connects the points situated highest in all the transverse sections of the wing. In other words, it is the line of the highest points in the camber. It will be seen, therefore, to commence with, that on the under side there is no camber at all at the root of the wing, that the camber rapidly increases until a point is reached just short of one- third of the length of the wing—in fact, it is little more than two-ninths; that from that point the camber is slowly decreased until it disappears altogether at a point at or about two-thirds of the length of the wing, and that the remaining third is perfectly flat, or so slightly curved that the pressure movements are practically identical with those on a flat plane. By examining and comparing the plan and section, it will be found that sections of maximum camber and chord coincide at the point already indicated, and that the centre of area of the whole wing is not far removed from this region. All points on the perimeter of the wing are on the same plane except just on the body side of the vertices of the parabolse, where the leading edge is slightly arched upwards so as to give a greater angle of incidence here than at any other part of the wing. In fact, everything that contributes to lifting effect is at a maximum just at this point. The entering edge is bluff near the body, tapering towards the tip. The line of highest points on the top camber is close to the leading edge at the body-end of the wing, but gradually recedes to the middle of the wing (lore and aft) near the tip. The details of top and bottom camber, &c, as described, are designed with the object of fixing, absolutely, the position of the centre of pressure so that disturbing effects cannot arise with variations of incidence within th» limits of ordinary flying angles. The tail is a large area non-lifting tail of short leverage, capable of being oscillated after the manner of a bird's tail. I also provide means for up and down movement, as with ordinary elevators, but I believe that this will no longer be necessary when the machine has passed the experimental stage. _ The manner of operating the wings for the purposes stated above— vi*., to maintain stability, to steer and to vary the speed of flight- will now be described. The spars on which the wings are mounted are connected to the body by an articulated joint so that they may be moved forwards and rearwards by rotation in the plane of their incidence to the wind ; FEBRDARY 7, 1914. in other words, when moved forwards they get a slightly upward dihedral set, and when moved backwards they get an inverted dihedral set. This fore and aft motion is independent of the motion by which the angles of incidence are varied. Fig. 2 (a) shows the plan form in ordinary flight; (b) shows the position in which the wings are placed for a-cent, and (f) for descent, while (d) illustrates in plan form the position which would be taken to make a right- hand turn. Fig. 2. In the same way, for a left-hand turn, the left wing should be retired. In (b) both wings have been advanced, and, as already explained, they have, therefore, a slight upwaid set. The centre of upward pressure has been brought to a position in front of the centre of gravity ; as well as this, the centre of resistance has been raised without being increased. These combined results cause the machine to ascend. In (c) the reverse has taken place—the wings have been retired, at the same time getting a downward set so that the machine is caused to descend. In (d) the right wing has been retired without alteration of the angle of incidence. This operation results firstly in causing the machine to bank owing to the shortening of the lever arm through which the upward pressure act-, and secondly in lowering the keel-plane in relation to the longitudinal axis of rotation and in moving it backwards in relation to the vertical axis of rotation. These results are most advantageous when turning. The machine, deflected by the " banking " which changes the direction of the aero-dynamic reaction on the planes, turns to the right. The rapidity of turning can be increased or decreased to any degree required by a corresponding retirement of the inside wing. Stability, both lateral and longitudinal, is controlled in the same way. If the machine be dipping, both wings are advanced ; if rising, both wings are retired. If the machine be canted to one side the wing on the rising side is retired, and so on. 49. In comparing this system with the warp and rudder system I should like to point out that : (a) In all the movements described, the static conditions which govern stability are maintained unimpaired. b) In turning on the wing control principle the inside wing is retired, which has the effect of shortening the pressure arm on that side, so that the centre of pressure moves towards the outer side of the curve ; the machine, therefore, tilts over until the e.g. is directly under the new position of the c.p. The " bank " is now, therefore, both caused and maintained by static conditions, and the method would be equally effective if all speed were lost, and the machine were dropping vertically through the air. The effect is not cumu lative in tendency, and absorbs no power from the engine. The turn is not made round a permanent centre fixed at the eg of the machine, but round a virtual or instantaneous centre on the line or the pressure produced, so that practically speaking the two wings move at equal speeds. It should also be noted that as the curve is generated by the translatory motion of the centre of gravity, and by that alone, the line of the propeller thrust must always remain tangential to the curve. (r) In changing direction upwards or downwards, the static conditions are also preserved. Direction is changed not by kinetic violence, but by a slight movement of the wings, which changes the relations of pressure and gravity—the weight of the machine acting through a tiny arm of a few inches is the agent that now changes the direction. It is clear, I think, that there can be very little violence or shock here. The machine gently swings into her new path. The system consists in readjusting the conditions of stability so that she automatically takes up a new position, in which she is just as stable as she was in the old one. Contrast the graceful movements of the birds with the jerky oscillatory turns usually made by aeroplanes even in the hands of the most expert aviators. 50. Remembering the balance of the keelplane round the centre of gravity, the upward and downward set of the wings which accompany advance and retirement respectively become of import ance. In turning in what is called " still air," danger may come 146
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